The present application claims priority to Japanese Application No. P10-205516 filed Jul. 21, 1998, which application is incorporated herein by reference to the extent permitted by law.
The present invention relates to an organic electroluminescence element including an organic compound which emits light when a current is injected thereinto, and particularly to an organic electroluminescence element suitably used for an ultrathin organic EL (Electroluminescence) display.
A display is generally classified into a fixed type Braun tube or CRT (Cathode Ray Tube) and a flat panel display developed for satisfying requirements toward portability and thinning. The Braun tube, which is high in luminance and good in repeatability of color, is frequently used at present; however, it is disadvantageous in that the occupied volume is large, the weight is heavy, and the power consumption is large. On the other hand, the flat panel display, which is lightweight and is superior in luminous efficiency to the Braun tube, is expected to be used for a display screen of a computer or a television set. At present, as the flat panel display, an active matrix drive type LCD (Liquid Crystal Display) is put into commercial production. The LCD has no spontaneous light emission function, and therefore, it displays an image with the aid of external light (backlight). As a result, the LCD is disadvantageous in that the viewing angle is narrow, the power consumption of the backlight becomes large when the LCD is used in a dark environment because of the lack of spontaneous light emission function, and the responsivity to a high definition and high speed video signal expected to be practically used in the future will be probably insufficient.
As a display capable of solving the above-described various problems, an organic EL display using an organic electroluminescence material which emits light when a current is injected thereinto becomes a focus of attention in recent years. The organic EL display, which is a spontaneous light emission type flat panel display without provision of the backlight, is advantageous in enlarging the viewing angle by the spontaneous light emission function of the organic EL display. The organic EL panel display is also able to reduce the power consumption still more because of its operational characteristic of lighting up only necessary pixels, and it may considered to ensure the responsivity sufficient to keep up with the above-described high definition and high speed video signal.
As an essential element of the organic EL display having the above advantages, there has been known an organic electroluminescence element having the following structure. An electrode layer patterned into stripes (anodes), formed of a transparent conductive film, is formed on a transparent substrate. An organic electroluminescence layer patterned into stripes is formed on the transparent electrode layer, and an electrode layer patterned into stripes (cathodes), formed of a metal thin film, is formed on the organic electroluminescence layer. That is to say, in this structure, the organic electroluminescence layer is held between the transparent electrode layer and the metal electrode layer, and the stripes of the transparent electrode layer and the metal electrode layer are crossed to each other to form a matrix structure. When a voltage is applied to selected one of the stripes of the transparent electrode layer and selected one of the stripes of the metal electrode layer, a current is allowed to flow in the associated one of the stripes of the organic electroluminescence layer, to make luminous the associated pixel.
The materials used for forming the organic electroluminescence layer are classified into a high molecular material formed by polymerizing a monomer, and a low molecular material. In the case of forming the organic electroluminescence layer using a low molecular material, a film of the low molecular material is formed by vacuum vapor-deposition. On the contrary, a film of a high molecular material cannot be formed by vacuum vapor-deposition, and therefore, in the case of forming the organic electroluminescence layer using a high molecular material, a film of the high molecular material is formed by painting a solution containing the high molecular material.
Various studies have been made in regard to the above-described organic electroluminescence layer made from a low molecular material. For example, C. W. Tang, S. A. VanSlyke, and others have developed a so-called single hetero type organic electroluminescence layer of a double layered structure including a thin film made from an organic compound having a positive hole transfer characteristic and a thin film made from an organic compound having an electron transfer characteristic, whereby positive holes and electrons injected in respective thin films from an anode and a cathode are re-combined with each other to cause light emission (Applied Physics letters, Vol. 51, No. 12, pp. 913-915, 1987). In this organic electroluminescence layer, the organic compound having the positive hole transfer characteristic or the organic compound having the electron transfer characteristic serves as an electroluminescence material, in which light emission occurs in a wavelength band corresponding to an energy gap between a base state and an excited state of the electroluminescence material. The organic electroluminescence layer of the double layered structure was effective to significantly reduce the drive voltage and to improve the luminous efficiency. After that, C. Adachi, S. Tokita, T. Tsutsui, S. Saito, and others have developed a so-called double hetero type organic electroluminescence layer of a three-layered structure including a thin film made from an organic compound having the positive hole transfer characteristic, a thin film made from an organic compound having an electroluminescence characteristic, and a thin film made from an organic compound having the electron transfer characteristic (Japanese Journal of Applied Physics, Vol. 27, No. 2, pp. L269-L271, 1988). Further, C. W. Tang, S. A. VanSlyke, C. H. Chen, and others have developed a structure in which an electroluminescence material is contained in an organic compound having the electron transfer characteristic (Journal of Applied Physics, Vol. 65, No. 9, pp. 3610-3616).
In an organic electroluminescence element having either of the above-described organic electroluminescence layers, however, since the metal electrode layer has a thickness of several hundreds nm or more and thereby light is specularly reflected from the metal electrode layer, there arises a problem in that it fails to obtain a sufficient contrast.
To solve such a problem, a method has been examined of reducing the reflection of light from the back side of the metal electrode layer by using a circular polarization sheet or coloring the transparent substrate; however, even in accordance with the method, a desired contrast cannot be obtained yet.
An object of the present invention is to provide an organic electroluminescence element capable of reducing reflection of light from a metal electrode layer, thereby ensuring a sufficiently high display contrast in the case where the element is used for a display.
To achieve the above object, according to a first aspect of the present invention, there is provided an organic electroluminescence element including: a first electrode layer; an organic electroluminescence layer formed adjacent to one surface of the first electrode layer; and a second electrode layer formed adjacent to the other surface of the organic luminescence layer, the second electrode layer having an optical reflectance to visible light specified in a range of 50% or less. The above optical reflectance can be set at a value in a specific range by adjusting the thickness of the second electrode layer.
With this configuration, since the second electrode layer formed adjacent to the other surface of the organic electroluminescence layer has an optical reflectance to visible light specified in the range of 50% or less, a display using such an element is able to display an image with a desirable contrast.
According to a second aspect of the present invention, there is provided an organic luminescence element including: a first electrode layer; an organic electroluminescence layer formed adjacent to one surface of the first electrode layer; a second electrode layer formed adjacent to the other surface of the organic electroluminescence layer, the second electrode layer having an optical reflectance to visible light specified in a range of 50% or less; and a low optical reflection layer formed on the second electrode layer on the opposed side to the surface adjacent to the organic electroluminescence layer. In this element, the low optical reflection layer is preferably composed of at least one or more layers including a layer having an optical reflectance to visible light specified in a range of 10% or less.
With this configuration, the second electrode layer has an optical reflectance to visible light specified in the range of 50% or less and also the low optical reflection layer having an optical reflectance specified, preferably, in the range of 10% or less is formed on the second electrode layer on the opposed side to the surface adjacent to the organic electroluminescence layer, a display using such an element is able to display an image with a desirable contrast.
According to a third aspect of the present invention, there is provided an organic electroluminescence element including: a first electrode layer; an organic electroluminescence layer formed adjacent to one surface of the first electrode layer; a second electrode layer formed adjacent to the other surface of the organic electroluminescence layer, the second electrode layer having an optical reflectance to visible light specified in a range of 50% or less; and a conductive layer formed on the second electrode layer on the opposed side to the surface adjacent to the organic electroluminescence layer in such a manner as to be in close-contact therewith and to have a shape along the surface of the second electrode layer, the conductive layer constituting an electrode portion in combination with the second electrode layer. In this element, the optical reflectance to visible light of the conductive layer is preferably in a range of 60% or less, more preferably, 50% or less.
With this configuration, since the second electrode has an optical reflection to visible light specified in the range of 50% or less and also the conductive layer is formed on the second electrode layer on the opposed side to the surface adjacent to the organic electroluminescence layer, a display using such an element is able to display an image with a desirable contrast.